Sheet metal remains the primary material utilized in the making of an automobile body, despite the advent of plastics technology. Sheet metal body parts, such as fenders and quarter panels, are formed utilizing conventional stamping presses. In an effort to minimize operating cost and maximize part quality, it is desirable to monitor operation of these presses, so as to ensure the process remains within statistical control. It would also be desirable to monitor operation of any machine having one tool or die member being movable relative to a part or to another die member, including extrusion machines, broaching machines, die casting machines, injection molding machines, etc. In die casting and injection molding, it would be desirable to monitor movement of the screw of plunger during injection. All of the above-mentioned processes are in need of improved process monitoring.
For example, U.S. Pat. No. 4,750,131, issued to Martinez, discloses a method of detecting non-conforming parts in a progressive die press having a plurality of forming stations, each of which includes a forming tool and a force transducer. The method includes the steps of producing a preselected number of sample parts while collecting reference force data from the force transducers for a plurality of displacements. The reference force data and the displacement data are applied to a processor which produces a reference curve for each of the forming stations. Production force data are provided to the processor and compared to the curve for each of the displacements and for each of the forming stations. A non-conforming part is indicated when the production force data for at least one of the displacements is different from the reference force data of the reference curve by a preselected difference.
U.S. Pat. No. 4,987,528, issued to O'Brien, discloses a signature analysis control system for a stamping press. A reference signature corresponding to desired force values at particular sampling points during the RAM cycle is developed. The force signature for successive cycles of the press is then compared to the reference signature, and the press inhibited in the event that any sampling point deviates from the corresponding point in the reference signature by more than predetermined limits.
However, these existing systems and methods for monitoring manufacturing processes are rigid and inflexible in that they rely on one aspect, i.e. force amplitude, to detect problems. For example, the '131 patent teaches collecting reference and production force data to obtain reference and production force curves. The production force curve is then compared to the reference force curve, and difference beyond acceptable limits indicates flaws. Similarly, the '528 patent discloses controlling press operation based on comparisons between desired force amplitude values and actual force amplitude values. Reference and operational force signatures are generated from force signals and compared to each other to detect out-of-tolerance conditions. By limiting analysis to force amplitude, however, many aspects of the process can not be properly monitored. As a result, the process can go out of statistical control, even though the force amplitude may be in control. The problem goes undetected until the unacceptable parts that have been formed roll off of the line. Similarly, the process may be in control even though the force data is outside the reference envelope in some non-critical areas.
Because there are many factors which affect process control and part quality, it is most desirable to monitor press operation utilizing a plurality of variables which can be extracted from the press signature in real time.